The development of optical scanning systems and associated electronic components has witnessed efforts towards miniaturization and better resolution through the aid of the improved products in the semiconductor industry, e.g. such as light sensitive photodiode arrays, injection laser diodes, etc. Because of their extremely small size these opto-electronic components may require one or more associated demagnifying lens systems to converge and focus an image beam into the extremely small area of the semiconductor photo responsive target. Where the photodetector-lens arrangement is employed in an optical scanning system, especially one involving rotational movement of the scanning optics, the distance between the image source and the photodetector target usually varies as the scan progresses, so that a continuous adjustment of the focussing of the image by the lens system on the photo responsive target is required. Often, this is achieved by an opto-electronic servo mechanism, which feeds back an error signal from the photodetector to a drive-focus controlling mechanism for the lens optics in an effort to maintain the image properly focussed on the target.
An exemplary environment where this need of continuous focus adjustment is necessary is in the lumber processing industry, e.g. a sawmill, where tree stems are examined for size and quality by an optical scanning system. The optical scanning system scans the stem from a distant viewing station and provides output signals representative of the characteristics of the stem, e.g. sweep, knots, etc. to electronic processing circuitry from which there is obtained, data representative of optimum partitioning of the log to maximize board feet yield and minimize waste. The optical scanning apparatus at the scanning station may be rotationally positioned above and to the side of the stem platform and may be arranged to sweep its imaging field of view along the length of the stem about a fixed axis of rotation. During the rotation of the viewing optics the distance between the portion of the stem being viewed and the photodetector changes, so that it is necessary to continuously compensate for the defocussing of the imaging lens that is positioned in front of the sensing opto-electronic module.
Previous proposals to deal with this problem have involved the use of a lens system having a large focal depth and an acceptance of the resultant defocussing as tolerable or the use of a servo arrangement for actively repositioning the lens system in response to an error feedback signal from the photodetector. Obviously the former approach creates a number of disadvantages, not the least of which is the less accurate data regarding stem partitioning and waste, while the use of an active servo mechanism adds components to the system that not only increase system complexity but add to its cost.